Mima Mounds

The Mima Mounds are a mysterious formation of thousands of mounds and are part of a 637-acre preserve. Each mound stands 6 to 8 feet tall and is covered with native grasses and flowers. The mounds have been a mystery since discovered by Capt. Charles Wilkes during a charting operation in the 1800s. He thought they were burial mounds, but found only earth inside.

Geologists and others have since developed several dozen hypotheses, including glacial freeze-and-thaw cycles, erosion, interplay between wind and vegetation, an earthquake or two, a tsunami or perhaps a volcanic eruption.

One popular theory credits generations of grubbing pocket gophers with creating the mounds while building underground dens. But there is nothing close to consensus.

Some other theories:

• Gravel, stones and soil that washed upon a melting ice-age glacier may have collected in pits called “suncups.” As the ice melted, the sediment pits settled in mounds.

• Earthquakes sent shock waves through the soil and moved the earth into mounds where the peak waves intersected.

• The mounds are deposits from sediment-rich floods from a glacially dammed lake. Some believe the mounds come from sediments deposited where water worked its way around vegetation.

One mound, inside a DNR gravel pit a few miles from the preserve, has been sliced open. The bottom two-thirds consists of the glacial outwash, pale and full of pebbles. The top third is made up of black silty earth that could be fine charcoal from repeated burning, many scientists believe.